1. Field of the Invention
The present invention relates to a communication system, and more particularly, to a method and apparatus for scanning channels in wireless local area networks.
2. Description of the Related Art
Wireless local area network (WLAN) technology is now popularly used in various applications. Numerous organizations devote extensive resources to research seeking improvements in WLAN data communication quality. In a WLAN, a wireless transmit/receive unit (WTRU) may be located within communication ranges of several access points (APs). However, the AP associated with the WTRU and the channel the WTRU operates on may change over time due to signal strength variations. On the other hand, when the WTRU roams among various APs, the channel utilized may be adjusted in accordance with a location of the WTRU and the signal quality. FIG. 1 illustrates a circumstance in which a station 13 roams from the communication range of an AP 11 to the communication range of an AP 12. For the roaming, before the station 13 can be associated with the AP 12, a handoff procedure has to be performed. The handoff procedure includes a scanning phase, an authentication phase and a re-association phase, and the scanning phase incurs significant delay.
According to the IEEE 802.11 standard, the scanning phase scans all channels in order to determine which channels a station can utilize. The two currently available modes for scanning are active scanning and passive scanning. For active scanning, a station selects a channel and sends a broadcast probe request frame and then waits a predetermined period of time. If no “probe response frame” is transmitted by the AP(s) in response to the probe request frame within the predetermined period of time, the station selects a next channel and repeats the procedure.
For passive scanning, a station simply goes to the channel of interest and passively listens for the periodic beacon frames sent out by AP(s) (if any). If no periodic beacon frame is transmitted by the AP(s) within a predetermined period of time, the station selects a next channel and repeats the passive scanning operation.
However, operations of transceiving packet will be interrupted whether active scanning or passive scanning is utilized. Take, for example, a dual-band, e.g. 2.4 GHz and 5 GHz, station that requires 140 ms to scan each channel. Such a dual-band station operating on 38 operation channels would need 5320 ms to scan all the channels. That is, the station has to stop communicating with an AP for 5320 ms in order to complete the operation of scanning all channels. Because the data forwarding and data receiving operations are interrupted during this period, important packets could be lost. Therefore, a new method for channel scanning procedures that improves lost-data rates while scanning is needed by the market.